What is the smallest extant animal species discovered? Of course the answer probably depends on precise definitions (of "animal," of "size," of how to determine representative dimensions of a species, etc.), which might make for a better thread anyway.

I haven't exactly researched the question, but when I search online the most common answer is the tardigrade, which is comparable to a large paramecium. That can't really be the smallest though, as it is much larger than the parasitic crustacean Stygotantulus stocki, comparable to a small paramecium.

A wise restriction for the contest would be that the animal has to be mature & capable of reproduction (whether by sexual or asexual means), otherwise we get into larvae and even embryos of species with external fertilization. And if it's colonial, the whole colony size counts, not just the individual coral polyps or whatever.

If it's poor form to bump extremely old threads, a mod can just fork this to a new one.

At some point the Wikipedia article on Stygotantulus was updated to reference this article about newly discovered species of crustaceans in the Tantulacus genus, with adult Tantulacus dieteri apparently only 85 μm long. That is very small indeed, and the WP author claims it is a "contender for the world's smallest arthropod," though this doesn't appear to be mentioned in the citation. It does seem like a newly discovered "smallest arthropod" would get at least a passing mention of that fact in the published article presenting its discovery.

Why isn't the answer to these questions easy to find? Surely there are people who care about this kind of thing. Obviously we will continue to find more competing species, but that's kind of the fun of it. Why isn't there any straightforward way to keep track of contenders for the smallest animal species yet discovered?

Eebster the Great wrote:At some point the Wikipedia article on Stygotantulus was updated to reference this article about newly discovered species of crustaceans in the Tantulacus genus, with adult Tantulacus dieteri apparently only 85 μm long.

That's surprisingly tiny, but the article (researchgate pdf) looks like it gives the figure for a larva rather than a sexually mature individual, which was still unknown. I know there are some other phyla where mature individuals are often only 100 μm or so, and some are a bit smaller. I found a reference to some gastrotrichs reaching only 70 μm, and some that give comparable sizes for certain rotifers. Female cycliophorans are bigger but males are only 30-40 μm, but they don't survive on their own so might not count.

But then beside semantic problems like that, how could you know if the very smallest are minute individuals or still growing? I think probably the reason the answers to these questions aren't easy to find is because they're not easy to determine, with a bunch of little animals near the same limit and no great way to decide a winner between them.

My first thought was what about that handful of single-celled mammalian parasites like devil facial tumour disease or canine transmissible venereal tumour? They're part of kingdom animalia but fail pretty much any other criterion for being an animal.

They are however able to survive and reproduce at incredibly small sizes

Eebster the Great wrote:At some point the Wikipedia article on Stygotantulus was updated to reference this article about newly discovered species of crustaceans in the Tantulacus genus, with adult Tantulacus dieteri apparently only 85 μm long.

That's surprisingly tiny, but the article (researchgate pdf) looks like it gives the figure for a larva rather than a sexually mature individual, which was still unknown. I know there are some other phyla where mature individuals are often only 100 μm or so, and some are a bit smaller. I found a reference to some gastrotrichs reaching only 70 μm, and some that give comparable sizes for certain rotifers. Female cycliophorans are bigger but males are only 30-40 μm, but they don't survive on their own so might not count.

That's pretty cool. 30 microns is pretty ridiculous for an animal, but then this paper reports a population of Myxozoans of the species Myxobolus szekeli produce spores that are just 8.7 × 4.1 μm, which is pretty insane when you think about it. Of course, that's just the spores, and the phylum also has a plasmodium stage, but it's still incredibly small. I think that might be a good phylum to consider in a search for the smallest animals.

Liri wrote:Also, "what is an animal?" is a surprisingly tricky question.

I think all the organisms discussed so far are unquestionably animals. Was there something specific you had in mind?

eSOANEM wrote:My first thought was what about that handful of single-celled mammalian parasites like devil facial tumour disease or canine transmissible venereal tumour? They're part of kingdom animalia but fail pretty much any other criterion for being an animal.

They are however able to survive and reproduce at incredibly small sizes

Would you really call transmissable cancer a "parasite"? By that logic, human prion protein has it beat by a mile.

eSOANEM wrote:My first thought was what about that handful of single-celled mammalian parasites like devil facial tumour disease or canine transmissible venereal tumour? They're part of kingdom animalia but fail pretty much any other criterion for being an animal.

They are however able to survive and reproduce at incredibly small sizes

Would you really call transmissable cancer a "parasite"? By that logic, human prion protein has it beat by a mile.

Prions wouldn't usually be considered "life" (cf viruses), these transmissible cancers are genetically distinct from the species they infect, and cannot reproduce with it meaning that they're arguably separate species. They engage in a non-mutual relationship with their host which does not necessarily kill their host which seems to make them some sort of parasite or possibly parasitoid.

Of course, I was being somewhat facetious when I said they might be the smallest animal, mostly to highlight the fact that so much of the terminology necessary to answer the question isn't sufficiently narrowly or consistently defined.

Yeah, the trouble is in the article; at what point does it become an animal? The cancer is definitely animal life and definitely parasitic, but it's not really an organism and doesn't have a reproductive mode that really helps to identify individuals.

It's more like a parasitic infestation than it is like an actual cancer that develops in the afflicted individual, but it's definitely not Sacculina, either, despite the latter having a stage that's similar in gross morphological terms.

So much depends upon a red wheel barrow (>= XXII) but it is not going to be installed.

It's probably - for me - more interesting/worthwhile to narrow the taxa in question a bit - what's the smallest vertebrate? Insect? Mollusk? Cause if we leave it at a broad definition of animal, we're gonna be down to a single cell which, while pretty cool, isn't really that interesting. Cause what's interesting about teeeeeeeny multicellular animals, especially more complex ones, is how they fit all their necessary machinery inside. Like, the smallest vertebrate is a competition between a few bat and frog species, which is awesome, cause they both have pretty neato morphologies.

Liri wrote:Also, "what is an animal?" is a surprisingly tricky question.

I think all the organisms discussed so far are unquestionably animals. Was there something specific you had in mind?

Similar things to what eSOANEM mentioned - single-celled dudes that are phylogeneticly within Animalia. That includes a lot of what were formerly called "protists".

Apart from Myxozoa (which is fascinating in its own right), are there other examples of this? I know that many protozoa were once considered animals (or plants, like all bacteria), but I can't personally think of the opposite case. I'm guessing these are mostly simple, derived, microscopic parasites?

Copper Bezel wrote:Yeah, the trouble is in the article; at what point does it become an animal? The cancer is definitely animal life and definitely parasitic, but it's not really an organism and doesn't have a reproductive mode that really helps to identify individuals.

It's more like a parasitic infestation than it is like an actual cancer that develops in the afflicted individual, but it's definitely not Sacculina, either, despite the latter having a stage that's similar in gross morphological terms.

The thing is, both you and Liri seem to agree that this cancer is definitely not an individual organism. I get the value of pointing out clever counterexamples, but do these sorts of distinctions actually matter in terms of what could realistically qualify as an acceptable answer to the topic? I mean, you seem to straightforwardly assume that viruses are definitely not alive, even though I suspect more people would defend the claim that virions or even prions were living individuals than tumors. Are transposons living?

Liri wrote:It's probably - for me - more interesting/worthwhile to narrow the taxa in question a bit - what's the smallest vertebrate? Insect? Mollusk? Cause if we leave it at a broad definition of animal, we're gonna be down to a single cell which, while pretty cool, isn't really that interesting. Cause what's interesting about teeeeeeeny multicellular animals, especially more complex ones, is how they fit all their necessary machinery inside. Like, the smallest vertebrate is a competition between a few bat and frog species, which is awesome, cause they both have pretty neato morphologies.

Those are interesting too, but the difference is that these can be much more easily found with a search. I'm trying to find the ungoogleable here. And unfortunately, restricting the taxa does not resolve the issue of transmissable cancers.

Copper Bezel wrote:Yeah, the trouble is in the article; at what point does it become an animal? The cancer is definitely animal life and definitely parasitic, but it's not really an organism and doesn't have a reproductive mode that really helps to identify individuals.

It's more like a parasitic infestation than it is like an actual cancer that develops in the afflicted individual, but it's definitely not Sacculina, either, despite the latter having a stage that's similar in gross morphological terms.

Functionally the transmissible cancers are pretty similar to bacteria in that any individual cell is reproductively viable but the tumour/infection can only thrive in a host of a certain species.

Like Eebster says, restricting taxa doesn't get us round such obviously ridiculous examples so we have to look to other restrictions. In particular, you could identify any number of typical animal traits and require them; for instance requiring sexual reproduction would rule out the transmissible cancers just fine but leave open pretty much anything "obviously" animal

Even Trichoplax has the capacity to reproduce in a controlled mode that a transmissible cancer lacks.

Eebster the Great wrote:The thing is, both you and Liri seem to agree that this cancer is definitely not an individual organism. I get the value of pointing out clever counterexamples, but do these sorts of distinctions actually matter in terms of what could realistically qualify as an acceptable answer to the topic?

I think the important thing for me to clarify is that no, I didn't think I was contributing anything useful on the main topic. = o Alien space squids, really.

Despite knowing that planktonic crustaceans are quite small, I'm actually surprised that there isn't a flatworm or something smaller, but it looks like the smallest are about twice the Tantulacus record.

So much depends upon a red wheel barrow (>= XXII) but it is not going to be installed.

eSOANEM wrote:Like Eebster says, restricting taxa doesn't get us round such obviously ridiculous examples so we have to look to other restrictions. In particular, you could identify any number of typical animal traits and require them; for instance requiring sexual reproduction would rule out the transmissible cancers just fine but leave open pretty much anything "obviously" animal

Unfortunately, this would rule out all bdelloids as well as other scattered species (and Trichplax adhaerans, which Copper Bezel metnioned). Sexual reproduction is certainly basal to animals, but not every animal species is still capable of it.